Superchargers are used to pump air into an engine at a greater rate than natural aspiration. Combustion air enters the supercharger at nearly atmospheric pressure. Rotors in the supercharger carry nearly atmospheric air, via low-pressure internal chambers, to an outlet port where the air is pressurized for delivery to the cylinders of an associated engine. The discharge of the nearly atmospheric air into the pressurized outlet creates backflow noise in the form of a pneumatic report or pop. When repeated at the high frequency which is typical of supercharger operation, this series of reports becomes a whine that may be undesirable in an automotive application.
Fixed backflow ports in supercharger housings have been utilized to reduce supercharger generated sound by allowing a small amount of pressurized air to flow from the pressurized outlet of the supercharger into the nearly atmospheric low-pressure internal chambers. The backflow of pressurized air into the chambers tends to lower the pressure differential between the outlet and the low-pressure internal chambers gradually so that when the chambers exhaust into the higher pressure outlet, there is less energy in each pulse, thereby reducing sound generated by the pressure differential. However, fixed ports are operable to reduce supercharger noise across a small range of engine speeds and are limited to a reduction of supercharger sound generated by the introduction of low pressure air transported by the supercharger rotors to the higher pressure outlet.
Accordingly, it is desirable to provide a supercharger having backflow ports that operate to reduce supercharger generated noise across a broad operating range of the supercharger.